While I agree that all dungeon levels being themed would get annoying after awhile, I also feel that there must be a way to have it both ways.
One option might be to have different rooms given themes, and then the set of themes that appear on the level decide which types of monsters show up. Tension rooms would contain monsters suiting the theme, of course. And then, within any particular room, monsters generated in that room would be more likely to match the theme. All other monsters would have a small chance of appearing, even if no appropriate theme is present.
The same theme patterns could also inform "basic" dungeon features, as well as specialty monster generation. For instance, an undead-themed room has some chance of containing one or two graves. If an undead-themed room and an orc-themed room are both generated on the same map, there would be a greater chance of orc skeletons, for instance.
Also, some rooms would just be generic rooms, which, among other things, would result in mixed tension rooms if a tension room is created in one.
These would be just like room properties in ADoM, in that they're inherent to the room. In fact, I think that rooms should have a variety of properties like this. I'd love to see "chaotic" rooms that are non-rectangular, for instance. If all rooms had "alignment", then that could influence altar generation, foe generation rates and types, corruption rates, etc.
Dungeon and floor themes could then also be included, to further alter the properties of the level. Using a variety of factors like that should ensure that the game feels more "real" without sacrificing variation and interest.